Support for specifying the internet-up domain in ClientOptions`

[mattheworiordan]

A customer that uses a vanity domain has asked whether or not it is also possible to create a CNAME for internet-up.ably-realtime.com. The aim is to minimise as far as possible the requests that go outside the site's top-level domain.

This was raised internally at https://github.com/ably/ideas/issues/434.

The viability of this proposal hinges on whether this is really needed and we can justify the additional work in our SDKs, monitoring, onboarding processes, etc.

┆Issue is synchronized with this Jira Task by Unito

[QuintinWillison]

See also this Slack thread (internal).

[lmars]

Perhaps we should add an internetAvailabilityURL option, with the requirement that it's a URL that returns a 200, then customer's who want to use their own domains can just point this at one of their own properties (e.g. https://example.com/health) rather than us needing to setup a CNAME for our own URL.

[mattheworiordan]

An alternative / additional option too could be to have internetUpCheckDisabled: true for environments where the Internet is not available (private networks).

[QuintinWillison]

For the private networks / no public Internet access use-case to be covered as well as the capability to override the internet-up endpoint location, I would propose that @lmars' suggestion of internetAvailabilityURL sounds like a good name, but that we make it nullable... so, as a client option:

• default: it has some value which is idiomatic for the SDK language that indicates to "use Ably's default":
  • for JS that could be undefined
  • for other languages, where only null is available, it's more likely to be idiomatic that a static string value https://internet-up.ably-realtime.com/ would be appropriate
• override: if set to a string value (including empty string) then it is parsed by the SDK as a URL and then used instead of the Ably default endpoint
• disable: if explicitly set to null then the Internet-up checks are disabled

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2777